Quote - Using box at 4 is not the best. 

Using sinc at 1 is not the best.

The best filter size depends on which filter you're using.

If you really want to know how all these work, read this thread thoroughly:

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=3559882

What I've found very useful for post filter is to not use it when I first render a scene, and then create a new scene where I plug the render into the background's alternate diffuse channel, remove all the lights, and then render at the same size. In this second render, which takens about twenty seconds, I use the post filter. (I also make sure to not use hsv or anything which changes the brightness of objects.)